In the United States someone legally becomes an adult when they turn 18 years old. At 18, U.S. citizens can vote. They can also serve in the military and go fight in wars for the country. An 18-year old can also buy cigarettes and other tobacco products, but 18-year-olds cannot buy alcoholic beverages. People in the U.S. need to wait until they are 21 to be able to do that.
At the age of 18, people can legally live on their own, work, and get married. However, turning 18 doesn't mean everyone is ready to take on adult responsibilities. At the age of 18, most people are just graduating from high school. Many will go to college. Some young people choose to go to college far from their homes and live in dormitories. Others pick a college close to home and stay with their parents to save money. Yet others will choose to live on their own. Legally, a parent does not have to provide for their child when the child turns 18.
Some parents in the U.S. take this very seriously and expect their children to move out as soon as they turn 18. Some parents will even forcibly kick their kids out when the kids turn 18. There tends to be some difference about this depending on if the kid is female or male. It is considered more acceptable for a female to stay living with her parents after her 18th birthday. Many females aren't expected to leave their parents' house until they get married. There is still an expectation for young men, however, to go out and live independent lives as soon as they can. With the economy of the U.S. struggling, some adult children are returning to their parents' home in their 20s and 30s. These are called boomerang kids, since like a boomerang, they return where they came from.